Abstract

Background

The major lineages of eusocial insects, the ants, termites, stingless bees, honeybees
and vespid wasps, all have ancient origins (≥ 65 mya) with no reversions to solitary
behaviour. This has prompted the notion of a 'point of no return' whereby the evolutionary
elaboration and integration of behavioural, genetic and morphological traits over
a very long period of time leads to a situation where reversion to solitary living
is no longer an evolutionary option.

Results

We show that in another group of social insects, the allodapine bees, there was a
single origin of sociality > 40 mya. We also provide data on the biology of a key
allodapine species, Halterapis nigrinervis, showing that it is truly social. H. nigrinervis was thought to be the only allodapine that was not social, and our findings therefore
indicate that there have been no losses of sociality among extant allodapine clades.
Allodapine colony sizes rarely exceed 10 females per nest and all females in virtually
all species are capable of nesting and reproducing independently, so these bees clearly
do not fit the 'point of no return' concept.

Conclusion

We argue that allodapine sociality has been maintained by ecological constraints and
the benefits of alloparental care, as opposed to behavioural, genetic or morphological
constraints to independent living. Allodapine brood are highly vulnerable to predation
because they are progressively reared in an open nest (not in sealed brood cells),
which provides potentially large benefits for alloparental care and incentives for
reproductives to tolerate potential alloparents. We argue that similar vulnerabilities
may also help explain the lack of reversions to solitary living in other taxa with
ancient social origins.